If it's not a Mach460 system, all you have to do is buy a correct adapter for you wiring harness. Then follow the diagram on that harness and the new head unit. Match up the wires in the correct order and plug it in. If you have a Mach460 system, you do it the same but to do it correctly, you have to buy this box that scosche (?spelling?) sells that lowers the output of the new stereo to a wattage that won't blow the factory amps.
__________________ C&L 80mm mass air and True Flow pipe, Mac CAI, BBK 65mm tb, Steeda non-piggy back ud pulleys, MGW shifter, RAM HD clutch and aluminum flywheel, 3:73 gears, 2.5" Dynomax o/r x pipe, 3" MAC cat back system w/ 3 1/2" tips, Kenny Brown(front and rear strut tower brace, caster plates, subframe connectors,extreme matrix brace,jacking rails, torque brace, lowering springs), Bilstein shocks, 03-04 Cobra(side scoops, rear bumper cover, top), 6pt roll cage, white FR500's, FMS 2000 Cobra R air dam. | 
1999 Mustang V6 stereo help 
